Low temperature multilayer ceramic circuit boards are known that are suitable for use with low melting temperature conductive metals, such as silver, gold and copper. They have a low thermal coefficient of expansion (TCE) and thus they may be compatible with silicon or gallium arsenide. The ceramic circuit boards are made from glasses that can be fired at low temperatures, i.e., less than 1000.degree. C. The multilayer circuit boards are made in known manner by admixing suitable finely divided glass particles with organic materials, including resin, solvents, dispersants and the like, and casting the resultant slurry as a thin tape called green tape. A circuit may be screen printed onto the green tape using a conductive ink formulation comprising a conductive metal powder, an organic vehicle and a powdered glass, usually the same or a similar glass to that used to make the green tape.
When more than one green tape is to be stacked, via holes are punched into the tape which are filled with a via fill ink, made with conductive material, an organic vehicle and a glass, which will provide electrical contact between the circuits on the various green tape layers. The patterned green tapes are aligned and compressed or laminated under pressure prior to firing.
More recently, the multilayer ceramic circuit boards have been adhered to a metal support substrate to increase the strength of the multilayer board. The support board has a metal core that is coated with a bonding glass that adheres the ceramic tapes to the support substrate during co-firing. The use of a bonding glass has another advantage in that it greatly reduces the shrinkage of the green tapes during firing in the x and y dimensions, so that most of the shrinkage occurs only in the z, or thickness, dimension. This means that the printed circuits can be made with closer tolerances. The glasses used for the green tapes must have a TCE matched to the metal support however, to prevent delamination or cracking of the fired glass. Mixtures of crystallizable and non-crystallizable glasses, optionally including inorganic fillers, are also known that have the desired TCE characteristics.
Up to the present time, when multilayer ceramic circuit boards are to include passive components such as resistors or capacitors, discrete components have been mounted to the top of the fired boards, using solder or epoxy type adhesives to adhere the components to the multilayer ceramic. The addition of these components increases the number of steps needed to make these circuit boards, i.e., the components must be aligned and adhered to the ceramic multilayer board, and connected to a source of power. Further, in order to accommodate a number of discrete devices, the multilayer boards have to be large. Thus the costs of making such boards is high.
It would be advantageous to be able to screen print passive components onto multilayer, low temperature co-fired ceramic circuit boards because the packing density can be increased, reducing the size and cost of the packaging. Using the recently developed low firing temperature glasses and a metal support board that reduce shrinkage in the x and y dimensions, screen printing of such components to tight tolerances, and high precision placement, become feasible. Further, because fewer interconnects need to be made, reliability would also be improved.
In a copending application filed concurrently herewith, the present inventors have interleaved two types of green tapes so that large numbers of green tapes can be stacked without shrinkage in two dimensions.
Thus it would be highly desirable to develop appropriate inks that can be screen printed onto green tape layers to form embedded capacitors to tight tolerances with high precision placement.